external drive unmounting

Thought I posted this yesterday but could not find it so here goes again. This is a powermac g4 imac with 10.4.11 and lately it will no longer stay mounted. switched ports and hub to direct and it continues to unmount. This one belongs to my daughter in a different home completely. Now, 2 of mine, at my home are doing the same thing. One is an intel 2006 macbook with 10.6.8 and the other is an early 2007 mini (intel) also with 10.6.8. The g4 backs up to a WD drive thru "SuperDuper". The 2 intel machines back up to a seagate. Cables, prayers, hammers, nothing works. Does a software hack have something to do with this?